The invention relates generally to the storage of cryogenic fluids and, more specifically, to an improved differential pressure gauge that performs real-time calculations of liquid volumes so that the gauge is easier to use and can be configured to various models of cryogenic containers.
Cryogenic liquids, such as nitrogen, argon, nitrous oxide, oxygen, carbon dioxide, hydrogen, and the like, liquify at extremely cold temperatures. Unique problems are encountered in handling and storing cryogenic liquids because the liquids undergo density changes at various storage pressures. A cryogenic storage system contains an insulated tank for containing a cryogenic liquid in a liquid space. Even though the tank is insulated, heat will enter the tank, causing the liquid cryogen to slowly vaporize to a gas and, as a result, causing the volume of liquid in the tank to diminish. This vaporization creates a pressurized head space in an upper portion of the tank.
Differential pressure gauges and sensors are well known in the art for aiding in monitoring the volumes of liquids. A differential pressure sensor senses the difference between a pressure at the head space of the tank, or head pressure, and a pressure at the liquid space of the tank, or liquid pressure, also known as column pressure. The liquid pressure is affected by both the pressure created by the head space of the tank and the pressure due to the weight of the liquid in the liquid space above the liquid space measuring point. By measuring the pressure difference between the pressure at the liquid space and the pressure at the head space, the differential pressure sensor senses the pressure solely attributable to the weight of the liquid. Typically, this pressure is measured either in pounds per square inch (psi), or in inches of water column.
By dividing the sensed differential pressure by the density of the liquid, the height of the liquid above the liquid space measuring point may be calculated. This liquid height can then be displayed on the gauge. Determining the volume of the liquid in the container is more difficult, however. Once the differential pressure has been measured, an operator must turn to a calibration chart, separate from the gauge, to determine the liquid volume. Calibration charts are also required in order to determine a total liquid weight, or an equivalent gas volume (typically measured in standard cubic feet). The relation between the differential pressure measured by the sensor and the liquid volume of the tank is affected by the tank shape, dimensions, and orientation, as well as the liquid density. Each calibration chart is therefore uniquely designed for a particular cryogenic tank model, tank orientation, type of cryogenic liquid and expected pressure of the liquid. The liquid density is a function of the liquid type and the state of its pressure. In order to determine a liquid volume level, the operator must procure an appropriate chart and use the differential pressure reading with the chart. Such calibration charts are awkward to use, and separate charts are required for different combinations of the factors listed above. This prevents efficient on-site monitoring of the liquid volume.
There is a need in the art for a method of determining, in real-time, a liquid volume using an on-site differential pressure gauge.
There is a further need in the art for a differential pressure gauge that does not require the use of calibration charts in order to determine a liquid volume.
These needs and others are met by an improved differential pressure gauge, which allows real-time calculations of liquid volumes based upon the reading of a differential pressure sensor, an absolute pressure sensor, and upon initial, one-time inputs by an operator. These inputs do not require the use of a calibration chart. The gauge can be configured to work with most cryogenic storage tanks. The gauge receives data from a differential pressure sensor, which senses the pressure difference between the head space and the liquid space of a cryogenic storage tank. The gauge includes a keypad, a micro-controller, and a data display for level and pressure. The gauge further receives data from a pressure sensor, which senses the pressure in the head space.
In operation, a user initially inputs programming information into the gauge, such as the dimensions of the tank, the orientation of the tank, the desired units of display, type of liquid, stratification constant and any zeroing out calibration values (not to be confused with an entry based upon a calibration chart). Once the user has input the necessary programming information, the input data is stored, preferably in a nonvolatile memory such as an EEPROM, and it is not necessary to input the information again. Only if the information needs to be changed (as would be required by replacing the tank or the type of liquified gas) is further user action required. The gauge contains stored information such as cylinder dimensional formulas, unit conversion formulas, and properties (such as liquid density) of the liquified gas specified by the user. The formulas and properties are stored in memory contained on the onboard computer.
To determine a liquid volume present at a particular instant, the differential pressure sensor sends an analog signal, corresponding to a differential pressure, to the onboard computer contained within the gauge. The analog signal is converted to a digital signal. The gauge analyzes this digital signal along with the initial input information from the user, and a signal from a pressure sensor using stored formulas and properties to calculate a liquid volume. The results are displayed on the device, or may be transmitted via telemetry to a remote device of the user""s choosing. Because a visit by a human operator to a site is now not needed just to ascertain liquid volume, site visits by supply trucks can be minimized and can be automatically triggered by the gauge detecting that a tank""s liquid volume has fallen below a predetermined level.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, wherein like characters represent like parts, provide a more complete understanding of the nature and scope of the invention.